April 1941 rolled around, and Kapitänleutnant Siegfried Freiherr von Forstner commissioned U-402, a type VIIC boat built by Danziger Werft in Danzig, and under the 3. Unterseebootsflottille (or 3rd "Combat" Flotilla). The crew's first success came in January 1942, when on the 16th he attacked and sank the 12,000-ton Britishmotor shipLlangibby Castle. Four months later in April, their second and third ships were sunk, and then a fourth smaller ship. U-402 and her crew came upon convoy SC 107, going through the escorts, and sinking three steamers and damaging another using only 5 torpedos. Returning again in four hours, they attacked once more, again in the presence of escorts. At the end of this attack, U-402 had scored another two British steamers. It was out of torpedoes, and so headed for home port.

Out on another patrol on February 7, 1942, Freiherr von Forstner attacked convoy SC 118 three times. During the course of the first two attacks, the crew managed to send two ships to the bottom, and damaged one more (a Norwegian motor tanker, the Daghild). On the third attack, only two hours after they finished the second, U-402 sank two more ships. For the great success of himself and his crew against convoy SC 107, Forstner was awarded the Knights Cross to the Iron Cross on February 9th.

In April, Forstner was awarded his last promotion to Korvettenkapitän. Come May 11th, he attacked convoy SC 118 and sent two more enemy ships to the ocean floor. Unfortunately, during their next patrol on October 13, 1943, U-402 was attacked and bombed by American aircraft off of the aircraft carrierCard. Most of the crew went down with the ship, and a few survived the attack, including Herr Kapitän Forstner. However, he died nine days later on the 22nd from the wounds that he had received during the attack. (Nota Bene: Some sources contradict this and indicate that U-402 went down with all hands.) The wreckage of U-402 still lies at the location at which it was attacked in the North Atlantic at 48.56N, 29.41W (in case you own a GPS).

Kapitän Freiherr von Forstner was one of the more successful U-Boat commanders of World War II, and was considered an ace (sinking more than 50,000 tons) with 62,582 sunken tons to his credit.